The present invention relates to an improved solar cell and a process for fabricating thereof. More particularly, the present invention relates to a solar cell including a p-n junction located near a non-illuminated surface of the solar cell and a process for fabricating thereof.
Solar cells are widely used because they convert easily accessible energy from a light source, such as the sun, to electrical power to operate electrically driven devices, e.g., calculators, computers, and heaters used in homes. FIG. 1 shows a cross-sectional view of a layered stack that makes up a conventional silicon solar cell 10. Conventional silicon solar cell 10 typically includes a p-n junction 24 sandwiched between a p-type base layer 18 and an n-type layer 16, which is located near an illuminated (front) surface 11. The term "illuminated surface," as used herein refers to the surface of a conventional solar cell that is exposed to light energy when the solar cell is active or under operation. Thus, the term "non-illuminated surface" refers to a surface that is opposite the illuminated surface. The basic structure of p-n junction 24 includes a heavily-doped (about 10.sup.20 cm.sup.-3) n-type emitter layer (n.sup.+) 16 at or near the illuminated surface 11 and disposed above a moderately-doped (about 10.sup.15 cm.sup.-3) p-type base layer (p) 18. Commercial embodiments of conventional solar cells typically include an optional antireflective coating 14 and a p.sup.+ layer 20 that is formed between p-type base layer 18 and p-type silicon contact 22.
A typical depth of p-n junction 24 from the top of the n.sup.+ emitter layer 16 measures about 0.5 .mu.m. A shallow front p-n junction 24 is desired in order to facilitate the collection of minority carriers that are created on both sides of p-n junction 24. Each photon of light that penetrates into p-type base layer 18 and is absorbed by base layer 18 surrenders its energy to an electron in a bound state (covalent bond) and thereby frees it. This mobile electron, and the hole in the covalent bond it left behind (which hole is also mobile), comprise a potential element of electric current flowing from the solar cell. In order to contribute to this current, the electron and hole cannot recombine, but rather are separated by the electric field associated with p-n junction 24. If this happens, the electron will travel to n-type silicon contact 12 and the hole will travel to p-type silicon contact 22.
In order to contribute to the solar cell current, photogenerated minority carriers (holes in the n.sup.+ emitter layer and electrons in the p-type base layer) should exist for a sufficiently long time so that they are able to travel by diffusion to p-n junction 24 where they are collected. The average distance over which minority carriers can travel without being lost by recombining with a majority carrier is called the minority carrier diffusion length. The minority carrier diffusion length generally depends on such factors as the concentration of defects in the silicon crystal (i.e. recombination centers) and the concentration of dopant atoms in the silicon. As the concentration of either defects or dopant atoms increases, the minority carrier diffusion length decreases. Thus, the diffusion length for holes in the heavily-doped n.sup.+ emitter layer 16 is much less than the diffusion length for electrons in moderately-doped p-type base layer 18
Those skilled in the art will recognize that the n.sup.+ emitter layer 16 is nearly a "dead layer" in that few minority charge carriers created in emitter layer 16 are able to diffuse to p-n junction 24 without being lost by recombination. It is desirable to have n.sup.+ emitter layer 16 that is shallow or as close to surface of emitter layer 16 as possible for various reasons. By way of example, a shallow emitter layer allows relatively few photons to be absorbed in n.sup.+ emitter layer 16. Furthermore, the resulting photogenerated minority carriers created in n.sup.+ emitter layer 16 find themselves close enough to p-n junction 24 to have a reasonable chance of being collected (diffusion length &gt;junction depth).
Unfortunately, in the conventional solar cell design, the depth of n.sup.+ emitter layer is limited and cannot be as shallow as desired. Metal from emitter contacts 12, especially those formed by screen-printing and firing, can penetrate into p-n junction 24 and ruin or degrade it. The presence of metal in the p-n junction 24 "shorts" or "shunts" the junction. Therefore, although a shallow and lightly-doped n.sup.+ emitter layer 16 is desired in order to enhance the current produced by the cell, in practice, however, n.sup.+ emitter layer 16 is relatively deeper and more heavily-doped than desired to avoid shunting p-n junction 24. Consequently, in conventional solar cells, the deep location of n.sup.+ emitter layer 16 compromises the amount of current produced by the cell.
What is needed is a structure and process for fabricating a silicon solar cell, which has a high minority carrier diffusion length, eliminates shunting of the p-n junction and does not compromise the amount of current produced.